A group of eukaryotic microorganisms traditionally classified as one of the five animal kingdoms. Although the name signifies primitive animals, some protozoa (phytoflagellates and slime molds) show enough plantlike characteristics to justify claims that they are plants. This apparent conflict may be reconciled to some extent by considering protozoa as descendants of microorganisms which preceded plants and animals. Such an assumption might account for Euglena gracilis, a green flagellate which can fix carbon dioxide (CO₂) in light as algae do but, unlike algae, can also carry on a plantlike fixation in darkness. In addition, E. gracilis can fix carbon dioxide by pathways characteristic of animals. Inclusion of such metabolically unspecialized organisms, along with certain photoautotrophic chlorophyll-bearing species, might imply that the phylum is partly an arrangement for taxonomic convenience rather than a phylogenetically homogeneous group.